ShellStream.Expect over-discards on undecodable bytes, driving ArrayBuffer.ActiveLength negative

Start with ShellStream.ExpectRegex in src/Renci.SshNet/ShellStream.cs and ArrayBuffer.Discard in src/Renci.SshNet/Common/ArrayBuffer.cs. Reproduce the undecodable-byte arithmetic, then trace the existing Discard call sites and verify that consumed input bytes are counted without making ActiveLength negative. Done means the Release-only failure no longer occurs and the over-discard is surfaced at its origin if it recurs.

Summary

ShellStream.ExpectRegex decodes the read buffer to a string, matches against that string, and then
discards the byte count of the re-encoded match:

var bufferText = GetString(_readBuffer.ActiveLength);
...
var returnText = bufferText.Substring(0, match.Index + match.Length);

_readBuffer.Discard(_encoding.GetByteCount(returnText));

GetByteCount(returnText) equals the number of buffer bytes returnText was decoded from only when
the decode was lossless. It is not, whenever the consumed region contained a byte the encoding could
not decode: GetString replaces each maximal invalid subpart with U+FFFD, which re-encodes to
3 bytes regardless of how many bytes it replaced.

So Discard is asked for more bytes than the match occupied. And ArrayBuffer.Discard only checks
that bound with a Debug.Assert:

public void Discard(int byteCount)
{
    Debug.Assert(byteCount <= ActiveLength, $"Expected {byteCount} <= {ActiveLength}");
    _activeStart += byteCount;

    if (_activeStart == _availableStart)
    {
        _activeStart = 0;
        _availableStart = 0;
    }
}

In a Release build the assert is gone, _activeStart overshoots past _availableStart, and
ActiveLength (_availableStart - _activeStart) goes negative. The _activeStart == _availableStart
reset cannot fire either, because the overshoot skipped equality — so the buffer never recovers.

The next iteration then calls GetString(_readBuffer.ActiveLength) with a negative length, and throws.

• https://github.com/sshnet/SSH.NET/blob/f099365c9d4cf2ade92b92c203bbb2b345d2cd74/src/Renci.SshNet/ShellStream.cs#L548
• https://github.com/sshnet/SSH.NET/blob/f099365c9d4cf2ade92b92c203bbb2b345d2cd74/src/Renci.SshNet/Common/ArrayBuffer.cs#L106

What it looks like in production

System.ArgumentOutOfRangeException: Non-negative number required. (Parameter 'count')
   at Renci.SshNet.ShellStream.ExpectRegex(TimeSpan timeout, Int32 lookback, ExpectAction[] expectActions)
   at <our shell wrapper>.ExecuteCommandAsync(String command)

Reproduced across several runs and several hosts, against Junos network devices, reading a large
configuration and then issuing a second command on the same ShellStream.

Note this is a Release-only symptom. Under Debug the assert fires instead, so a test run would
report the over-discard at its origin rather than as a later crash.

Minimal repro of the arithmetic (no SSH server needed)

The defect reduces to this identity. With the default Encoding.UTF8:

foreach (var bytes in new[]
         {
             new byte[] { 0x61, 0xF8, 0x62 },       // 'a', a lone 0xF8, 'b'
             new byte[] { 0x61, 0xE2, 0x82, 0x62 }, // a truncated 3-byte sequence
             new byte[] { 0x61, 0xFF, 0xFF, 0x62 },
             new byte[] { 0x61, 0xC3, 0xB8, 0x62 }, // valid UTF-8, for contrast
         })
{
    var text = Encoding.UTF8.GetString(bytes);
    Console.WriteLine($"in={bytes.Length} out={Encoding.UTF8.GetByteCount(text)}");
}

in=3 out=5     <- Discard() is asked for 2 bytes more than the match occupied
in=4 out=5     <- 1 byte more
in=4 out=8     <- 4 bytes more
in=4 out=4     <- lossless decode, correct

Every case where the two disagree drives ActiveLength negative by the difference.

ExpectRegex is the only one of the ten Discard(...) call sites in ShellStream that computes its
count by re-encoding; the others count in byte space (ActiveLength, indexOf…, bytesRead) and are
not affected.

Suggested direction

The number of bytes to discard is a fact about the input bytes, so re-encoding the output string
cannot recover it. Options, roughly by intrusiveness:

• Decode with a Decoder and track how many input bytes produced the consumed prefix.
• Keep a char-index → byte-offset mapping while building bufferText.
• Independently, consider making ArrayBuffer.Discard clamp or throw rather than Debug.Assert, so an
  over-discard surfaces where it happens instead of as an ArgumentOutOfRangeException one call later.

Happy to test a patch — we have hosts that reproduce it reliably.

Environment

• SSH.NET 2026.0.0. The two lines above are byte-identical in 2025.1.0, 2026.0.0 and develop.
• .NET 10, Release build, Linux container.